Fleetsatcom System

The FLTSATCOM System
By Larry Van Horn, N5FPW

Copyright 2007 by Monitoring Times magazine and the author.Reproduction is not permitted with permission of the copyright holders. This article orgonally appeared in the January 2007 Milcom column in Monitoring Times magazine.
"I take great pleasure in inaugurating...the first satellite of the fleet satellite communications system. The gap is filled." With that message sent by Admiral James L. Holloway III, then Chief of Naval Operations in 1978, the FLTSATCOM system became operational.

FLTSATCOM (pronounced FleetSatCom, for Fleet Satellite Communications) is a versatile, high-capacity worldwide military communications system operated by the United States. NASA launched all eight of these spacecraft for the military services, all on Atlas/Centaur vehicles. With the launch of FLTSATCOM-8, there is six FLTSATCOM satellites in orbit, with two operational.

FLTSATCOM platforms provide instant communications between the President and commanding officers in the United States and remote units stationed anywhere in the world.

In addition to the Ultra High Frequency (UHF) capability of the earlier satellites in this series, FLTSATCOMs 7 and 8 also carry an Extremely High Frequency (EHF) communications package. This package has served as a test bed for the MILSTAR system terminals.

The Navy portion of the FLTSATCOM shared system provides communications between naval aircraft, ships, submarines, and ground stations. The Air Force portion of each satellite is part of the USAF Satellite Communications System (AFSATCOM). AFSATCOM links the National Command Authority with Strategic Command units, and other arms of the Air Force. Each satellite contains the communications package outlined in Table 1. Table II is a synopsis of the FLTSATCOM satellite launches to date. Table III is a detailed breakdown of the UHF communications packages aboard each FLTSATCOM. FLTSATCOM bandplans use the designators A (Alpha), B (Bravo) and C (Charlie).

Launch History

FLTSATCOM-1, launched in February 1978, provided service from Southeast Asia across the Pacific to the West Coast of the United States from its 172 deg East orbital slot. FLTSATCOM-2, launched in May 1979, was initially positioned at the 23 degree West orbital slot. After the launch of of FLTSATCOM-3, this satellite was moved to the 72 degree East orbital slot to cover the Indian Ocean area from Africa to the Phillippines.

FLTSATCOM-3, launched in January 1980, provided service from the middle of the United States across the Atlantic and the Mediterranean. FLTSATCOM-4, launched in October 1980, was co-located with FLTSATCOM-1 to provide coverage over the Pacific. FLTSATCOM-5, launched in August 1981, was damaged during launch, and was never declared operational. FLTSATCOM-7 (aka USA 20), launched in December 1986, was placed in orbit co-located with FLTSATCOM-1. It now provides service over the United States from its 100 degree West orbital slot. FLTSATCOM-6, was lost after being struck by lightning shortly after launch in March 1987. FLTSATCOM-8 (aka USA 46), launched in September 1989, was placed over the Atlantic at the 23 degree West orbital slot.

Launch Profile/On-Orbit Operations

FLTSATCOMS were launched on Atlas-Centaurs rockets from Launch Complex 36 on the Cape Canaveral Air Force Station in Florida. After launch, the Atlas stage completed its burn and fell into the ocean. The first burn of the Centaur stage injected the spacecraft into a parking orbit, at a perigee altitude of about 148 kilometers (92 statute miles) and apogee of approximately 369 kilometers (229 statute miles). After a coast period of about 14 minutes, the Centaur engines were ignited again and they placed the spacecraft into a highly elliptical, or egg-shaped, "transfer orbit" with an apogee of about 35,988 kilometers (22,362 miles).

Once in this transfer orbit, the Centaur stage released the spacecraft and, as its final act, performs a retro-maneuver which took it safely out of the flight path. The U.S. Air Force Space Systems Division (SSD) then assumed command of the satellite, operating through its Consolidated Space Test Center (CSTC) at Onizuka Air Force Base, Sunnyvale, California. NASA tracking stations throughout the world, together with the Air Force Satellite Control Network Remote Tracking Stations, provided range and range-rate measurement support to assist the CSTC controllers in bringing the satellite on station.

The elliptical transfer orbit was designed so that the satellite would reach its apogee while over the equator. To convert the orbit from an elliptical to a circular one, and change the angle of inclination so that the flight path would be more nearly above the equator, CSTC operators correctly aimed the spacecraft then fired an onboard solid propellant motor at a selected apogee. This final burn "transferred" the satellite into a circular "drift" orbit, almost at synchronous altitude and with the angle of inclination reduced to 5 degrees. The FLTSATCOM then drifted into its assigned place in the global network, where the CSTC controllers fired small thrusters of the onboard hydrazine reaction control system to stop the drift motion.

When the satellite was located above and in line with the equator at an altitude of about 35,789 kilometers (22,238 miles), and a velocity of 11,071 kilometers (6,879 miles per hour), the movement becames "synchronized" with that of the Earth below. Thus the satellite appears to remain stationary in the sky, while actually completing one orbit every 24 hours. All fully geosynchronous satellites, including those for commercial communications, weather observation, and military communications, are stationed in this type of orbit above the equator at the same altitude, spaced around a circle about 266,000 kilometers (165,000 miles) in circumference. They are carefully separated by distance or by assigned radio frequencies to prevent interference between their individual communications systems.

Once on station the FLTSATCOM satellites were inclined to the equator, so that it appears from the ground to be moving back and forth from north to south. At the same time, it appears to ground observers to move slightly east and west from the center point of the orbit, tracing a constant figure "8" across the equator in the sky.

Satellite Description

FLTSATCOM-7/8, with an 37 kilogram (81pound) adapter for connection to the vehicle, weighed about 2,296 kilograms (5,061 pounds) on the ground, and had a mass of about 1,223 kilograms (2,696 pounds) in space after burning up the apogee motor propellants. Both satellites measure 13.2 meters (43.4 feet) from tip to tip of the fully extended solar panels. The main body is 2.3 meters (7.5 feet) wide, and 6.6 meters (21.6 feet) high from the bottom of the body to the tip of the offset spiral antenna mast. Both the spiral antenna and the solar panels were in a retracted configuration for launch, as was the 4.9 meter (16 foot) diameter, silver-filled stainless steel mesh UHF antenna.

The main body consists of three attached hexagonal modules called the payload module, the spacecraft module, and the EHF module, or FEP (Fleet EHP Package). The solar arrays extend from the spacecraft module, which also contains the hydrazine-fueled reaction control system thrusters andtanks, Sun and Earth sensors, a reaction wheel which spins to hold the spacecraft steady in its operating attitude, and the other systems needed for control and operation of the spacecraft.

The payload module contains the three antenna systems, the transponders for the 23 UHF channels, and all the associated electronics required to support communications functions. The offset mast is the UHF transmit antenna. A small, separate conical helix antenna atop the central mast serves as the S-band Tracking, Telemetry and Control (TT&C) antenna, and is used to command and monitor the spacecraft on 2202.5, 2252.5, and 2262.5-MHz. The superhigh frequency antenna horn protrudes through a hole cut into the UHF antenna mesh.

The FEP also contains the Extremely High Frequency communications package, with its 30 (maximum) voice channels. The package was designed and built by the MIT Lincoln Laboratory in Lexington, Massachusetts. The EHF antenna, consisting of a 5 degree steerable spot beam and an Earth coverage aperture, that look through cutouts in the center portion of the UHF transmitting antenna.

In operation, the momentum wheel provides a means to control the spacecraft attitude so that the antennas are always aimed at the Earth. The two solar arrays rotate on their extended arms so that they constantly face the Sun. These two arrays contain three panels each, with a total of 23,000 solar cells, each 2 by 4 centimeters (0.79 by 1.57 inches) in size, that produced about 2,200 watts at the beginning of their orbital life. Three 24-cell nickel-cadmium batteries provide power when the spacecraft must operate in the Earth's shadow; 2,150 of the solar cells are reserved for battery charging.

This aging satellite constellation has been providing service to the fleet for well over 28 years and was supplemented in the 1980s with the LEASAT constellation of satellites.

Note: The Fleet Satellite Broadcast Subsystem has 15 subchannels of encrypted message traffic at an input data rate of 75 bps per channel. These subchannels are time-division multiplexed and transmitted in a one-way RF transmission at 1200 bps. The shore-based terminal transmits this data on a direct sequence spread-spectrum SHF signal to the UHF satellites, where the signal is translated to UHF and down-linked to the subscribers. The queued and/or channelized message traffic for Fleet Satellite Broadcast transmission is encrypted and inputted to a time-division multiplexer, where it becomes a 1200-bps data stream and is passed to the transmitter. The structure of the Fleet Satellite Broadcast transmission allows 15 subchannels: eleven 75-bps subchannels for general-service message traffic, two 75-bps subchannels for special-intelligence message traffic, and two 75-bps subchannels for Fleet weather data. A sixteenth subchannel in the Fleet Satellite Broadcast transmission is used for frame synchronization.

Note: AFSATCOM 5-kHz channels 11-17 are regenerative, which means that the uplink RF signal at 317-MHz containing 75 bps messages, is converted to baseband; the message bits are amplified, reshaped, and remodulated and transmitted on the downlink at 243 MHz. Processing limits the signal to 75 bps and requires a special radio. AFSATCOM 5-kHz channels 18-22 are non-regenerative as there is no processing done other than the conversion. AFSATCOM is specifically designed for emergency action message (EAM) dissemination, force direction, force report back and Commander-in-Chief (CINC) internetting. The AFSATCOM terminal segment consists of all Air Force airborne and ground communication equipment, required interfaces, and related terminal equipment.

Note: Two operating modes are used on these UHF channels. The narrowband mode is limited to a 5-kHz bandwidth (a single 5-kHz channel, or a 5-kHz bandwidth on a 25-kHz or 500-kHz channel). The wideband mode is limited to a 25-kHz bandwidth (a single 25-kHz channel, or a 25-kHz bandwidth on a 25-kHz or 500-kHz channel).

Copyright 2007 by Monitoring Times magazine and the author. Reproduction is not permitted without permission of the copyright holders. This article originally appeared in the January 2007 Milcom column in Monitoring Times magazine.